1. Field of the Invention
The present invention relates to a driving device for driving a thermal element, and more particularly to a driving device used in a thermal printing apparatus having a thermal printing head, such as a thermal printer and a thermal printing facsimile machine.
2. Description of the Prior Art
A well known thermal printing head is formed on a ceramic substrate and has many thermal elements arranged in a line on the substrate. The head length is the same as the ISO A4 paper width. It has 8 dots per mm heating resistor (thermal element) density. If the size of a heating resistor is 100 (.mu.m).times.200 (m), the number of heating resistors is 1664. These resistors are in general divided into a plurality of small groups (e.g. 32 resistors/group). When one group has 32 heating resistors, 52 driving devices are required for deriving 1664 thermal dots. In order to realize high-speed printing, driving devices are provided on the ceramic substrate together with heating resistors.
A driving device receives the printing information and applies a driving current to the respective resistor according to the received information. The resistor through which a driving current flows generates thermal energy. In this manner, a black character or other information may be printed on a heat sensitive paper.
A conventional driving device has a receiving circuit for receiving information and a latch circuit for temporarily storing the received information. The latch circuit is used to realize high-speed printing operation. That is, by inserting the latch circuit between the receiving circuit and a driver circuit, an input operation to the receiving circuit can be executed in parallel with an output operation from the latch circuit to the driver circuit. In other words, when a driving current is supplied to a heating resistor according to the information in the latch circuit, a next new information to be printed can be set in the receiving circuit at the same time. However, it is to be noted that two timing control signals are required, each to control a receiving operation and a latch operation, respectively.
In a high-speed printing operation, the idle time when no driving current flows to a heating resistor is short. Where the nonconducting period is short and the black information requires consecutive energization of the same heating resistor, the heating resistor becomes overheated. As a result, a printed pattern is not clear. Moreover, the heating resistor may be damaged and even destroyed.
In order to avoid these problems, a driving device having two latch circuits has been proposed. A first latch circuit is used to store the information to be printed, which is transferred from the receiving circuit. A second latch circuit is used to store the information which has just been printed. The current conducting time is controlled according to contents of both the first and second latch circuits. If a black information is stored in the first latch and a white information is stored in the second latch, a first predetermined period is given as a current conducting time. If a black information is stored in the first latch and a black information is stored in the second latch, a second predetermined period shorter than the first predetermined period is given as a current conducting time. Thus, overheating of the same heating resistor can be avoided.
However, since the second latch circuit must be added, a new timing signal to latch the information of the first latch circuit into the second latch circuit is necessary. Therefore, three timing signals (the receiving timing signal, the first latch timing signal and a second latch timing signal) are required and are applied to the driving device through signal lines formed on a ceramic substrate. In addition, control signals for controlling current conducting times are required. Therefore, many signals are applied to the driving device through the respective signal lines. In general, electrical connecting signal lines and the driving device are made by using bonding wires. As described above, since the driving device must input many signals, many bonding wires must be used.
As previously discussed, a thermal printing head generates a large amount of heat energy, so that the driving device located near the head is significantly affected by the generated heat energy. Since bonding wires are not thermostable, the reliability of the connections is low. Therefore, a small number of bonding wires is better. However, if timing signals increase by one, 52 additional bonding wires are required on the substrate in the above case.
Moreover, wide signal lines are preferred, in order to accommodate high-speed signal transmission, and gold or other precious metal is used in such signal lines. Therefore, a small number of signal lines is better in a low cost thermal printing head and a low cost driving device.
Electrical terminals of the driving device may be connected to the signal lines on the substrate by means of a direct-bonding technique, for example a flip chip assembly technique, instead of wire-bonding. However, reliability of directly bonded connecting portions is low, just as with wire-bonding, and the number of the timing signals cannot be reduced.